docs: def get_rlzs_by_gsim_grp(self, sm_lt_path=None, trts=None):
          """
          :returns: a dictionary src_group_id -> gsim -> rlzs
          """
          self.rlzs_assoc = self.get_rlzs_assoc(sm_lt_path, trts)
          dic = {grp.id: self.rlzs_assoc.get_rlzs_by_gsim(grp.id)
                 for sm in self.source_models for grp in sm.src_groups}
          return dic